While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, DUV and VUV lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using an ArF excimer laser as the light source is thought requisite to the micropatterning technique capable of achieving a feature size of 0.13 μm or less.
The ArF lithography started partial use from the fabrication of 130-nm node devices and became the main lithography since 90-nm node devices. Although lithography using F2 laser (157 nm) was initially thought promising as the next lithography for 45-nm node devices, its development was retarded by several problems. A highlight was suddenly placed on the ArF immersion lithography that introduces a liquid having a higher refractive index than air (e.g., water, ethylene glycol, glycerin) between the projection lens and the wafer, allowing the projection lens to be designed to a numerical aperture (NA) of 1.0 or higher and achieving a higher resolution. See Journal of Photopolymer Science and Technology, Vol. 17, No. 4, p 587 (2004).
In the photolithography using an ArF excimer laser (wavelength 193 nm) as the light source, a high sensitivity resist material capable of achieving a high resolution at a small dose of exposure is needed to prevent the degradation of precise and expensive optical system materials. Among several measures for providing high sensitivity resist material, the most common is to select each component which is highly transparent at the wavelength of 193 nm. For example, polyacrylic acid and derivatives thereof, norbornene-maleic anhydride alternating copolymers, polynorbornene, ring-opening metathesis polymerization (ROMP) polymers, and hydrogenated ROMP polymers have been proposed as the base resin. This choice is effective to some extent in that the transparency of a resin alone is increased.
Studies have also been made on photoacid generators. In prior art chemically amplified resist compositions for lithography using KrF excimer laser, photoacid generators capable of generating alkane- or arene-sulfonic acid are used. However, the use of these photoacid generators in chemically amplified resist compositions for ArF lithography results in an insufficient acid strength to scissor acid labile groups on the resin, a failure of resolution or a low sensitivity. Thus these photoacid generators are not suited for the fabrication of microelectronic devices.
For the above reason, photoacid generators capable of generating perfluoroalkanesulfonic acid having a high acid strength are generally used in ArF chemically amplified resist compositions. These photoacid generators capable of generating perfluoroalkanesulfonic acid have already been developed for use in the KrF resist compositions. For instance, JP-A 2000-122296 and U.S. Pat. No. 6,048,672 (or JP-A 11-282168) describe photoacid generators capable of generating perfluorohexanesulfonic acid, perfluorooctanesulfonic acid, perfluoro-4-ethylcyclohexanesulfonic acid, and perfluorobutanesulfonic acid. JP-A 2002-214774, US Patent Application Publication 2003-0113659 A1 (JP-A 2003-140332), and US Patent Application Publication 2002-0197558 A1 describe novel photoacid generators capable of generating perfluoroalkyl ether sulfonic acids.
On the other hand, perfluorooctanesulfonic acid and homologues thereof (collectively referred to as PFOS) are considered problematic with respect to their stability (or non-degradability) due to C—F bonds, and biological concentration and accumulation due to hydrophobic and lipophilic natures. The US EPA adopted Significant New Use Rule, listing 13 PFOS-related chemical substances and further listing 75 chemical substances although their use in the photoresist field is excluded. It has already been proposed to apply the Rule to perfluoroalkanesulfonic acids and derivatives thereof, summing to 183 chemical substances.
Facing the PFOS-related problems, manufacturers made efforts to develop partially fluorinated alkane sulfonic acids having a reduced degree of fluorine substitution. For instance, JP-A 2004-531749 describes the development of α,α-difluoroalkanesulfonic acid salts from α,α-difluoroalkene and a sulfur compound and discloses a resist composition comprising a photoacid generator which generates such sulfonic acid upon irradiation, specifically di(4-tert-butylphenyl)iodonium 1,1-difluoro-2-(1-naphthyl)-ethanesulfonate. JP-A 2004-2252 describes the development of α,α,β,β-tetrafluoroalkanesulfonic acid salts from α,α,β,β-tetrafluoro-α-iodoalkane and sulfur compound and discloses a photoacid generator capable of generating such a sulfonic acid and a resist composition comprising the same. JP-A 2002-214774 discloses such photoacid generators having difluorosulfoacetic acid alkyl esters (e.g., 1-(alkoxycarbonyl)-1,1-difluoromethanesulfonate) and difluorosulfoacetic acid amides (e.g., 1-carbamoyl-1,1-difluoromethanesulfonate) although their synthesis method is lacking. Furthermore, JP-A 2005-266766 discloses a photosensitive composition comprising a compound capable of generating a partially fluorinated alkane sulfonic acid having a sulfonylamide structure derived from perfluoroalkylene disulfonyl difluoride.
The substances disclosed in these patents have a reduced degree of fluorine substitution, but suffer from several problems. They are less degradable because they are based on substantially undegradable hydrocarbon skeletons and they do not possess readily degradable substituent groups such as ester groups. A certain limit is imposed on the molecular design for changing the size of alkanesulfonic acid. The starting materials containing fluorine are expensive.
JP-A 2007-145797 discloses C1-C20 alkanecarbonyloxy or arenecarbonyloxy-1,1,3,3,3-pentafluoropropanesulfonates, typically triphenylsulfonium 2-(adamantane-1-carbonyloxy)-1,1,3,3,3-pentafluoropropanesulfonate. The control of diffusion is still insufficient.
JP-A 2007-161707 discloses photoacid generators capable of generating partially fluorinated alkanesulfonic acids having a polycyclic hydrocarbon group. In our experiments, they are poorly soluble in resist solvents and thus practically unacceptable.
With respect to the immersion lithography, there remain some problems. Minute water droplets are left on the resist and wafer after the immersion exposure, which can often cause damages and defects to the resist pattern profile. The resist pattern after development can collapse or deform into a T-top profile. There exists a need for a patterning process which can form a satisfactory resist pattern after development according to the immersion lithography.
In forming fine feature size patterns, the problem of pattern density dependency, that is, the size difference between isolated and grouped patterns having different optical contrast becomes significant. Using a photoacid generator capable of generating an acid with low diffusion, the problem of pattern density dependency can be overcome to some extent, but not to a satisfactory extent. Since a certain variation of exposure dose can occur in the practical fabrication of microelectronic devices, the resist composition is required to have an exposure latitude that maintains a substantially identical pattern profile even on a certain variation of the exposure dose. While the resist composition is required to achieve a further reduction of the pattern rule as well as a good balance of sensitivity, substrate adhesion, and etching resistance, it is also required to ameliorate the pattern density dependency and exposure latitude without sacrifice of resolution.